1. Field of the Invention
The present invention relates in general to exhaust emission control systems of an internal combustion engine, and more particularly to the exhaust emission control systems of a type that is suitable for a diesel engine.
2. Description of Related Art
An exhaust emission control system for a diesel engine is shown in Laid-open Japanese Patent Application (Tokkaihei) 7-189654. For collecting particulates (PM: particulate matter) in exhaust gas from the diesel engine, the known control system has a diesel particulate filter (DPF) with a catalyst, that is installed in an exhaust gas passage extending from the engine. Under operation of the engine, a differential pressure between upstream and downstream positions of the diesel particulate filter is monitored and, when the differential pressure exceeds a given degree, a throttle valve of the engine is so controlled as to lower an excess air ratio of the exhaust gas. With this, the exhaust gas temperature becomes increased thereby to burn off the particulates left in the filter.
After studying the above-mentioned known control system, the following facts have been revealed by the applicants, that are latently possessed by the known control system and appear when the diesel engine is under cold operation. That is, under cold operation, the cooling water and cylinder walls of the engine show a low temperature, and thus fuel for the engine fails to have a satisfied vaporization. Thus, if, for the purpose of increasing the exhaust gas temperature, lowering of the excess air ratio (that is, enriching the air/fuel mixture) is carried out when the engine is still cold, the exhaust gas from the engine is forced to have a large amount of hydrocarbon (HC) as compared with a case wherein the engine is sufficiently warmed. Furthermore, under such cold operation of the engine, a filter bed of the diesel particulate filter (DPF) has a low temperature, and thus until the time when the filter bed shows a sufficient temperature for a normal oxidization function of the filter, a large amount of hydrocarbon (HC) is inevitably emitted to the open air without being purified. These phenomena will be well understood when referring to the time charts of FIGS. 5A, 5B, 5C, 5D and 5E that show various data with respect to an elapsed time.
In order to solve the above-mentioned undesirable phenomena, a measure may be thought out wherein until the diesel particulate filter (DPF) shows its sufficient oxidization function, raising of exhaust gas temperature is carried out while controlling the amount of hydrocarbon (HC) exhausted from the engine. However, if lowering of the excess air ratio is made small, raising of exhaust gas temperature becomes small. In this case, the time needed until the diesel particular filter (DPF) shows its satisfied oxidization function is increased. Thus, also in this measure, a satisfied reduction of hydrocarbon (HC) emitted to the open air is not expected. This measure will be well understood when referring to the time charts of FIGS. 6A, 6B, 6C, 6D and 6E that show various data with respect to an elapsed time. For comparison, the data in case of the control system of Laid-open Japanese Patent Application 7-189654 are also shown in the time charts by thinner curves.
Although the above description is directed to the undesired phenomenon that takes place when, with the engine being under cold operation, the excess air ratio is lowered for increasing the exhaust gas temperature to reactivate the diesel particulate filter (DPF), viz., to remove the particulates (PM) in the filter, similar undesired phenomenon would take place in a case wherein, with the engine being under cold operation, the exhaust gas temperature is increased to reactivate a NOx trapping catalytic converter, viz., to remove S-poisoning from the catalyst of the converter.
It is therefore an object of the present invention to provide an exhaust emission control system of a diesel engine, which can increase the exhaust gas temperature without deteriorating the quality of the exhaust gas from the engine at the time when, with the engine being under cold operation, reactivation of the diesel particulate filter (DPF) and/or reactivation of the NOx trapping catalytic converter is needed.
According to a first aspect of the present invention, there is provided an exhaust emission control system of a diesel engine, which comprises an exhaust gas purifying device arranged in an exhaust gas passage extending from the engine, the exhaust gas purifying device having at least one of two functions, one function being a function of trapping particulate matter (PM) of the exhaust gas flowing in the exhaust gas passage and the other function being a function of trapping nitrogen oxides (NOx) in the exhaust gas when the exhaust gas shows a higher excess air ratio and reducing the trapped nitrogen oxides (NOx) to purify the same when the exhaust gas shows a lower excess air ratio; a three-way catalytic converter arranged in the exhaust gas passage upstream of the exhaust gas purifying device, the converter purifying hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gas when the excess air ratio of the exhaust gas is approximately 1 (one); a hydrocarbon adsorbing/releasing member arranged in the exhaust gas passage upstream of the three-way catalytic converter, the member selectively adsorbing thereon hydrocarbon in the exhaust gas and releasing the adsorbed hydrocarbon into the exhaust gas in accordance with a temperature of a catalyst bed of the three-way catalytic converter; and a control unit which is configured to carry out, upon need of reactivation of the exhaust gas purifying device under cold operation of the engine, lowering the excess air ratio of the exhaust gas when the temperature of the catalyst bed of the three-way catalytic converter is lower than or equal to a first predetermined temperature; raising the excess air ratio of the exhaust gas when the temperature of the catalyst bed of the three-way catalytic converter is higher than the first predetermined temperature but lower than or equal to a second predetermined temperature; and lowering the excess air ratio of the exhaust gas until the temperature of the exhaust gas purifying device becomes to a third predetermined temperature when the temperature of the catalyst bed of the three-way catalytic converter is higher than the second predetermined temperature.
According to a second aspect of the present invention, there is provided a method of controlling an exhaust emission control system of a diesel engine, the control system comprising an exhaust gas purifying device arranged in an exhaust gas passage extending from the engine, the exhaust gas purifying device having at least one of two functions, one function being a function of trapping particulate matter (PM) of the exhaust gas flowing in the exhaust gas passage and the other function being a function of trapping nitrogen oxides (NOx) in the exhaust gas when the exhaust gas shows a higher excess air ratio and reducing the trapped nitrogen oxides (NOx) to purify the same when the exhaust gas shows a lower excess air ratio; a three-way catalytic converter arranged in the exhaust gas passage upstream of the exhaust gas purifying device, the converter purifying hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gas when the excess air ratio of the exhaust gas is approximately 1 (one); and a hydrocarbon adsorbing/releasing member arranged in the exhaust gas passage upstream of the three-way catalytic converter, the member selectively adsorbing thereon hydrocarbon in the exhaust gas and releasing the adsorbed hydrocarbon into the exhaust gas in accordance with a temperature of a catalyst bed of the three-way catalytic converter. The method comprising, upon need of reactivation of the exhaust gas purifying device under cold operation of the engine, lowering the excess air ratio of the exhaust gas when the temperature of the catalyst bed of the three-way catalytic converter is lower than or equal to a first predetermined temperature; raising the excess air ratio of the exhaust gas when the temperature of the catalyst bed of the three-way catalytic converter is higher than the first predetermined temperature but lower than or equal to a second predetermined temperature; and lowering the excess air ratio of the exhaust gas until the temperature of the exhaust gas purifying device becomes to a third predetermined temperature when the temperature of the catalyst bed of the three-way catalytic converter is higher than the second predetermined temperature.